Solid wood is generally understood to be lumber or timber, that is pieces of wood sawn from a tree. This is distinct from engineered wood or manufactured wood products which include MDF, plywood, and OSB. Engineered wood is made of fibers or strands that are pressed or glued, there is more in the final stock than just wood (I.e., glue, lignin separated via heating and pressing).
The Enbrighten Zigbee switches I installed in some rooms work exactly as you note, if the zigbee network collapses for some reason, they still control the outlet or light they are hard wired to like any other dumb switch. I think for hardwired switches this is the normal behavior. For anything like Hue or Ikea Tradfri switches that are battery powered I think it highly depends on what they are connected to.